This invention relates to secure printing apparatus such as is employed in postage meters in which a print head utilised for printing secure information is removably mounted on the secure printing apparatus.
Secure printing apparatus such as postage meters include electronic means for carrying out accounting functions in respect of postage values which it is desired to apply to mail items by operation of a printer. The electronic means also carries out control functions for operation of the postage meter including operation of the printer. The operation of postage meters must be carried out in a secure manner to prevent unauthorised access to critical circuits of the postage meter whereby postage indicia could be printed fraudulently. Accordingly the postage meter is housed in a secure housing which prevents unauthorised access to the postage meter or if the secure housing is breached provides indication that tampering has occurred or has been attempted. Previously the print head provided in the postage meter to print the postage indicia has been housed in the secure housing so that not only is unauthorised access to the postage meter circuits prevented or inhibited but unauthorised access to the print head and electrical connections from the postage meter to the print head is also prevented or inhibited.
Previously postage meters have been provided with a drum printer or a thermal transfer printer for printing the postage indicia. With the drum printer, ink for printing the postage indicia is supplied by means of a replaceable absorbent roller containing liquid ink which rolls in contact with print dies on the print drum. With thermal transfer printers, ink is supplied as a layer on a replaceable ribbon which is fed past a thermal print head for transfer of ink to the mail items. Both the ink roller and the ink ribbon are removable from the postage meter by a user of the postage meter for replacement by a new ink roller or ink ribbon respectively. With both of these types of printer, the printer per se is maintained secure by the secure housing. In the case of the drum printer, mechanical elements for setting the printing elements of the printer are not accessible by a user of the postage meter and in the case of a thermal transfer printer, electrical connections to the print head for control and operation of the print head are protected from access thereto.
It is now proposed, instead of drum printers or thermal transfer printers, to use ink jet printing devices. Ink jet print heads are already used widely in computer output printers where security of operation thereof is neither a problem nor required. The ink jet print heads manufactured and sold for use in computer output printers and commonly available comprise a replaceable module including a row of ink jet nozzles and means for ejecting selectively ink from those nozzles. The module also includes electronic circuits for operation of the ink ejection means and an ink supply to supply ink to the nozzles to replenish ink ejected from the nozzles in printing. When the ink in the ink supply becomes depleted the entire module including the nozzles and electronic circuits is removed and replaced by a new ink jet print head module. Since the print head must be replaced relatively frequently due to depletion of the ink supply it would be inconvenient to require replacement of the print head to be carried only by authorised service personnel in a secure manner. It is desirable to be able to use standard commonly available manufactured ink jet print heads and to permit replacement thereof to be carried out without any necessity of breaching security of the secure printing apparatus so that the print head can be replaced by users of the secure printing apparatus.